In disc devices that record and reproduce optical discs such as CDs or DVDs, the laser light is irradiated onto a recording surface through an optical pickup including an objective lens, and the signal that is recorded on the disc is read by converting the reflected light therefrom to an electrical signal. The optical pickup is arranged so as to be moveable in the focusing direction (vertically) and the tracking direction (left/right) in response to current flowing in the coils of an actuator; during reproduction the optical pickup must maintain a condition in which the focal point is located on the recording surface.
To achieve this, such a disc device is constructed so as to perform a focusing servo action whereby the objective lens position is finely adjusted in response to the current flowing in response to the amount of reflected light detected by a photodiode, called a detector, to which the reflected light from the recording surface is guided after being received by an objective lens and resolved by a beam splitter. For example, utilizing the fact that, if the light is out of focus, the main spot will not be circular, four equal zones may be defined centered on the main spot and the differences between the amounts of light, of the amounts of light in the zones, in prescribed pairs of these zones may be used as focusing error signals, which are then detected. In this way, control may be performed so as to pass currents to the coils with voltages corresponding thereto such that the optical pickup is moved in the direction such as to cancel these errors.
Such a focusing servo action is generally called a focusing pull-in action. However, this focusing pull-in action can only be performed in the range in which the focusing error signal can be detected i.e. in a narrow range before and after the position of the focal point. Consequently, in such disc devices, a construction is adopted such that the objective lens is moved vertically, called focus searching, and the focusing servo action is turned on at the time point where a focusing error signal is obtained (see Japanese Patent Application Laid-open No. H5-182206, Japanese Patent Application Laid-open No. H5-166195, Japanese Patent Application Laid-open No. H11-120599 and Japanese Patent Application Laid-open No. 2000-339704).
However, as described above, when the focusing servo action is performed, the amplitude of the focusing error signal for driving the actuator must be amplified to a prescribed value. The amplification factor of this focusing error signal is called the focus gain. For example, in order to raise the reproduction capability in respect of a disc using disc wobble, the focus gain during reproduction is set to a gain adapted to disc wobble.
Such setting of the focus gain is performed for example during focus searching that is performed respectively when changing over from the stopped condition to the disc reproduction condition of the reproduction device and when focus is lost due for example to scratches or vibration, and during layer jumping, that is performed on changing over the layer that is reproduced in a multi-layer disc such as a DVD.
For example, the focus gain setting procedure that is performed during focus searching will be described with reference to the flow chart of FIG. 6. Specifically, a focus pull-in action is performed (step 602) after commencing focus searching from a focus servo action off condition (step 601). After the focus pull-in action, a focus servo on condition is produced (step 603) and the other servos (spindle servo, tracking servo, and sled servo) are turned on (step 604), and disc reproduction is then performed (step 605). The ordinary gain value A is employed as the focus gain value during the above steps 601 to 604.
The focus gain setting procedure during layer jumping will be described with reference to the flow chart of FIG. 7. Specifically, when layer jumping is performed (step 702) from the condition in which the disc is being reproduced (step 701), using the focus gain value A that is used in the current layer, the focus gain value B on the previous occasion when the actuator drive circuit effected a shift to this layer is input, causing the set value A of the focus gain to be replaced by the new focus gain value B (step 703).
A focus pull-in action is performed using this focus gain value B (step 704), after which the focus servo is put in the on condition (step 705), the other servers are put in the on condition (step 706) and disc reproduction of the layer to which a shift has thus been effected is performed (step 707).